In the production of formed coke, the process in which coal is formed with an added binder has been established on a commercial scale, while a process where agglomerated coal is carbonized has not yet been successful on such a scale so as to be useful for the amounts and quality required for use in a large blast furnace. This is due to the difficulty in the production of high quality formed coke on an industrial scale without causing crushing, agglutinating and cracking of the agglomerated coal that might occur depending on the heating and loading conditions in the carbonization process.
The present invention provides an effective process for carbonizing agglomerated coal by which agglomerated coal retains its shape through the continuous carbonization process on an industrial scale and, at the same time, the caking property of the raw material coal is utilized to improve the strength of the formed coke.
The inventors of the present invention carried out detailed and systematic investigations to determine the effect of heating and mechanical loading on the behavior of the agglomerated coal during carbonization and also on the strength and other qualities of the formed coke by use of a so-called carbonization oven simulator in which the conditions of heating and mechanical loading can be arbitrarily chosen. As a result, the range of heating rate as shown in FIG. 1 as measured at the center of the agglomerates is desirable. These data provide useful information for determining the most favorable heating conditions to obtain the best quality of coke and the lowest cost of production considering all the possible phenomena involved in the carbonization oven on the industrial scale, as is evident from the foregoing experimental techniques.
In particular, the upper and the lower limits of the heating rate at which the temperature at the center of agglomerated coal is maintained between 200.degree. and 400.degree. C. have been chosen for assuring the best conditions to improve the strength of the formed coke by keeping the velocity with which coal particles are softened and melted to each other, occurring from the surface towards the center of agglomerated coal, higher than a certain value. At the same time, unfavorable phenomena such as crushing, agglutinating and surface cracking of the agglomerated coal in the carbonization process are prevented.
These data are completely new and were discovered by the present inventors as a result of systematic investigations.
There has been a qualitative knowledge that, when agglomerated coal is heated in the temperature range above 400.degree. C., cracking may be formed on the surface owing to re-solidification and shrinking of the agglomerates. This fact has been quantitatively established in the present invention. The method of carbonization involving the heating speed at a temperature close to the mentioned upper limit is desirable from the point of view of the efficiency of the equipment. The carbonization of the present invention has been accomplished based on the entirely new discoveries found from the investigation of the heating rates with which the temperature of the center of agglomerated coal is increased from 200.degree. to 1000.degree. C. as shown in FIG. 1. The desirable heating rate in FIG. 1 depends naturally on the method of production, size, composition of raw materials and the initial temperature in the carbonization oven. However, the pattern of the curves as a whole and the basic principle remain unaltered.
Even if the entirely new pattern of heating (FIG. 1) is known, it remains still very difficult by the conventional techniques of carbonization of coal to apply the knowledge to the continuous carbonization on an industrial scale. It is evident to use, in the continuous carbonization of coal or agglomerated coal, an upright type, for example, a Lurgi, carbonization oven using a gas as heating medium. However, there exists no carbonization oven capable of satisfying the complicated heating rate characteristics as shown in FIG. 1.
Generally speaking, when complex characteristics are required for the heating rate, some carbonization ovens are employed in series to satisfy the need. But it is usually accompanied by technical problems such as handling of high temperature coal and sealing of high temperature gases. Some alternative methods have been proposed for adjusting the heating speed. Thus, a blow of cooling gas is applied to a part of the carbonization zone where a relatively slow heating speed is required, and a fraction of the heating gas is blown outside of the oven. However, this involves increased complexity in the installation, and prohibits an increased scale of the installation.
To solve these problems, the present inventors investigated the variation of thermal properties of agglomerated coal, such as, the specific heat and thermal conductivity during the carbonization process, and the heating treatment of the agglomerates with a gas as heating medium from the theoretical and experimental aspects. The present inventors have developed the new technic for the oven operation in accordance with the new pattern of heating. The features of the present invention lie in controlling both temperature and velocity of the flow of hot gas which are supplied to a tuyere at the middle and the lower parts of the carbonization zone in an upright type carbonization oven in such a way as to satisfy the experimental requirements as shown in FIG. 1.